"Once you start down the dark path, forever will it dominate your destiny".
If people falling and being redeemed are not so rare as to be unheard of, you'd think Yoda and Obi-wan would know. Their advice to Luke would be less "Seriously, you have to kill them both" and more like "eh, people fall and get redeemed all the time. Just talk about the power of friendship or something."
The redemption of Anakin works best if it isn't initially considered possible. It explains why the Jedi were so extreme in their selection and training of youngsters - because once you fall, you aren't coming back. The dangers of darksiders is so great that it warrants the 'stealing' of children. Whether or not the ship has sailed is not the point, the point is that the ship is a bad idea in general.